Vaccination against COVID-19 aims to produce an initial immune response to enable the body to quickly respond to attack and destroy SARS-CoV-2 whenever the vaccinated individual is later infected with the virus. All vaccines aim to expose the body to an agent, called an antigen, to provoke an initial immune response. In the initial immune response, the vaccinated individual produces antibodies and effector lymphocyte cells. Both the antibodies and the effector lymphocyte cells attack and destroy the antigen – in this case, the SARS-CoV-2 virus. Two major types of lymphocytes involved in vaccination response are the B cells, which produce antibodies, and the T cells, which directly attack the antigen. After the initial immune response following vaccination, the effector lymphocytes become memory cells, which remain in the body and retain the ability to attack the antigen upon a subsequent infection.
Vaccines against COVID-19 are designed based on an understanding of the SARS-CoV-2 viral structure. Like all viruses, the SARS-CoV-2 virus is a simple structure. It consists a core encased by a protein envelope. The core consists of the genetic code material ribonucleic acid (“RNA”) combined with nucleocapsid protein. The envelope is comprised of membrane protein, envelope protein, and spike (“S”) protein. The S proteins indeed resemble spikes and stud the virus envelope, giving the virus its crown- or corona-like appearance. The spikes are essential to the process whereby the coronavirus invades its host. Once the virus enters the human body, the spikes attach to cellular receptors, allowing the virus to enter the cell, begin to replicate, and cause the symptoms of COVID-19 disease in the human host.
Vaccines against SARS-CoV-2 infection use several platforms. The SARS-CoV-2 vaccine platforms under development include the following: inactivated virus; protein-based; viral vector; and nucleic acid-based. An inactivated virus is treated with chemical compounds that render it inactive but still able to stimulate the body’s immune system to respond. Protein-based vaccines involve injecting fragments of the SARS-CoV-2 protein envelope into the body. Viral vector and nucleic acid-based viruses are developed by genetic engineering.
Genetic engineering techniques facilitated rapid development of SARS-CoV-2 vaccines. Genetically-engineered viruses are based on either viral RNA or deoxyribonucleic acid (DNA). Molecular biologists use the viral RNA code as a template to create the DNA for DNA-based vaccines. In designing genetically-engineered vaccines against COVID-19, scientists create DNA or RNA polymers that code for the SARS-CoV-1 S protein. RNA vaccines are encased in a lipid capsule. DNA vaccines can be given as part of an adenovirus “vector.” An adenovirus is a type of virus that causes mild respiratory infections. Using genetic engineering techniques, DNA is incorporated into the adenovirus. Both encapsulated RNA vaccines and adenovirus vector vaccine particles are given by IM injection. The DNA or RNA from these genetically engineered vaccines is able to enter the subject’s cells. Within the subject’s cells, the DNA or RNA codes for the viral S protein, which is released into the systemic circulation. The body then initiates an immune response to the viral protein which it synthesized internally.
Clinical trials of vaccines against COVID-19 are designed to establish vaccine safety and effectiveness. In the US, drug development proceeds through a preclinical and clinical phase. Preclinical studies are conducted (1) with laboratory reagents or tissue cell cultures (“in vitro” or “test tube”); and (2) in experimental animals, usually mice, rats, and nonhuman primates. Clinical trials are designated as “early” or “late” stage. Early stage Phase I trials use single-doses in healthy normal subjects; involve single doses of the vaccine; and focus on safety. Early stage Phase II trials establish effectiveness in addition to safety and can use single or multiple doses. The late stage Phase III trials are large adequate and well-controlled trials required by law to establish safety and effectiveness for vaccine licensing. Current thinking is that it is not ethical to challenge subjects with exposure to COVID-19 after vaccination. Finally, for vaccine development, both Phase II and Phase III trials include placebo controls. Informed consent is crucial to these trials. Subjects should be fully informed of the benefits and risks of the trial and should be able to decide not to participate in the trial or withdraw at any time.
Several published reports of completed Phase II clinical trials of COVID-19 vaccines showed good safety and effectiveness. ModernaTX (“Moderna”), CanSino, and Oxford University (“Oxford”) published in peer-reviewed clinical journals results of Phase II trials of COVID-19 vaccines. All three vaccines were genetically engineered and coded for the SARS-CoV-2 S protein. Moderna used an RNA vaccine. CanSino used a nonreplicating Ad5 adenovirus vector DNA vaccine. A5 is a type of adenovirus that causes mild respiratory infections in humans. Oxford’s vaccine is a nonreplicating adenovirus vector, consisting of DNA delivered by the ChAdOx1 adenovirus, which infects chimpanzees. The trials enrolled from 45 to over 1000 healthy male and female subjects from 18 to 60 years of age and included both treatment groups and placebo groups. Moderna and Oxford gave two doses; an initial dose and a booster 28 days later. CanSino gave single doses. Follow-up was to 56 days. The most common local adverse events were pain and tenderness at the injection site. The most common systemic adverse events were headache, muscle ache, chills, fatigue, joint pain, and fever. The majority of adverse events were mild-to-moderate and resolved within several days. Oxford found that giving acetaminophen for the first 24 hours after dosing reduced many of the adverse events. All three vaccines produced strong T cell responses and high antibody titers against the S protein and against live SARS-CoV-2 virus. Vaccines produced higher antibody responses than convalescent plasma samples from recovered COVID-19 patients. However, antibody titers against SARS-CoV-2 virus in senior subjects were about half those observed in younger subjects.
The World Health Organization (“WHO”) maintains a website that tracks worldwide trials of vaccine candidates against COVID-19 and lists 6 Phase III trials in progress. The vaccine candidates in Phase III are from Moderna, AstraZeneca/Oxford, Pfizer/BioNTech (“Pfizer”), Sinovac, and Sinopharm. The Pfizer vaccine is an RNA-based vaccine. The Sinovac and Sinopharm vaccines are chemically-inactivated viruses. Many scientists believe that inactivated virus vaccines induce a stronger immune response than genetically engineered vaccines. The Moderna and Pfizer Phase III trials are taking place in US. AstraZeneca/Oxford will conduct one Phase III trial in the UK and one in Brazil. The Sinovac trial will also take place in Brazil. The Sinopharm trial will take place in the United Arab Emirates. The Phase III trials will be run at multiple clinical sites and will enroll from 2,000 to 30,000 healthy male and female subjects, of ages ranging from 18 years to “over 70.” Subjects will receive initial and booster vaccinations. The Moderna trial will enroll health care workers at high risk of contracting COVID-19. All other trials will use subjects in the community. All trials will investigate safety, anti-SARS-CoV-2 antibody titers in serum, and T-cell responses in blood. Subjects will be monitored for 1-2 years after immunization. A primary endpoint in the trials will be the percentage of subjects who contract COVID-19 in the community.
As per the WHO website, 10 different Phase II trials and 10 different Phase I trials of COVID-19 vaccine candidates are in progress. Companies running Phase II trials are Anhui of China; the Institute of Medical Biology of China; Inovio of South Korea; Takara Bio of Japan; Cadila of India; Genexine of South Korea; Novavax of Australia; Kentucky Bioprocessing of the US; Arcturas of Singapore; and Janssen/Johnson & Johnson of the US. Platforms include protein subunit, inactivated virus, DNA-based, RNA-based, and adenovirus vector. Primary endpoints are safety; secondary endpoints are antibody titers and T-cell responses. Companies running Phase I trials are based in Russia, Australia, the UK, Belgium, China, Canada, and Taiwan. Platforms include adenovirus vector, protein subunit, and RNA. A novel platform being tested in Phase I by Medicago of Canada is plant-derived virus-like particles, which are thought to produce a strong immune response despite being plant-derived. The WHO website also states that, as of July 31, 2020, 139 vaccine candidates are in in preclinical development worldwide.
The FDA is facilitating rapid availability of COVID-19 vaccines for the US. The Agency posted a Guidance for Industry on how to develop vaccines for licensing. The FDA has stated that it may consider granting an Emergency Use Authorization (“EUA”) for vaccines. Under an EUA, a vaccine would be made available to the public even though it has not yet undergone all the clinical testing normally required for licensing. The FDA has also stated that it will consider granting licenses for COVID-19 under the Accelerated Approval program before all the Phase III testing is complete. For both an EUA and Accelerated Approval, the vaccine must show preliminary evidence of safety and effectiveness in humans and strict criteria must be met.
Ethical issues arise once a vaccine is available. For example, if initial quantities are limited, this raises questions of prioritization for distribution. Priority might be granted to health care workers and other essential workers. Priority also might be granted to the more vulnerable populations, such as the elderly and those with pre-existing conditions. An additional question is how low-cost vaccines can be made available to all.
In summary, a tremendous worldwide effort is underway to rapidly develop a safe and effective vaccine against COVID-19 disease. Vaccines are based on a variety of different platforms. The most rapid progress has been made with genetically engineered vaccines. Several developers showed good vaccine safety and effectiveness in early-stage clinical trials and are now conducting late-stage clinical trials. Many other vaccine candidates are being actively studied in early-stage clinical trials. In vitro and animal studies are underway for even more vaccine candidates. Government and industry are collaborating to expedite vaccine availability, and it is conceivable that a vaccine against COVID-19 could be available by late 2020-early 2021. It is hoped that government and industry will also collaborate effectively to make affordable vaccines available to as many individuals as possible.